In case anyone's interested, I've been hacking like mad on my toy proxy since New Years'. Check out PersonalWebProxy for current downloads. It's got primitive forms of proxy filters, browser-based UI, RDF-based metadata management, logging, config, plugins, and some other goodies. So far, the major plugins include:

a noisy logger;

a content archiver that captures and saves all response headers and content in a directory structure loosely based on the requested URL;

an initial metadata harvester that fills up the RDF database with triples based on headers and details encountered during browsing

It works, and does stuff, but I'm sure it demonstrates a complete lack of understanding of large portions of the Twisted framework, Python itself, and likely causes forest fires. So, I hope many people will download it, snicker at it, and maybe set me straight on a few things and contribute a few patches and plugins.